Prefabricated roof truss systems have been used in construction for a number of years. Roof trusses can be made from wood, wood composites, metal and other materials.
In some situations, vertical arching or deflection of the roof truss occurs. The vertical arching of trusses appears to result from a difference in the temperature that the top and bottom chords are subjected to. In climates where it is necessary to insulate the building for efficient heating or cooling, the ceiling is generally insulated by placing the insulating material between and upon the bottom chords of the roof truss. Therefore the bottom chords are at least partially insulated and can be at a temperature near to that of the interior of the building. The top chords are left uninsulated and are subject to the ambient temperature and humidity. This temperature differential can cause different contraction and expansion of the various parts of the roof truss, resulting in vertical deflection of the roof truss.
Typically, the ceiling finish, usually a sheet material such as gyproc, is directly attached to the underside of the bottom chords of the trusses. A similar finish is typically attached to the interior partition walls. The ceiling finish typically abuts the wall finish. Therefore, if there is vertical arching of the trusses, the ceiling finish is lifted by the bottom chords, and cracks develop between the interior wall finish and the ceiling finish.
Cracking caused by vertical arching of the bottom chords of the trusses can be minimized by stiffening the bottom chord, but this makes the truss much heavier and more difficult to handle during construction and it is not always effective. As well, alternative methods of attaching the edges of the ceiling finish to the walls rather than the ceiling have also been developed. However, these methods require the installation of awkward and complicated fasteners on the construction site, increasing installation time and expense.
Therefore, there is a need in the art for a means of mitigating cracking caused by vertical arching of the bottom chords of trusses.